sshd_config

NAME

sshd_config - OpenSSH SSH daemon configuration file

SYNOPSIS

/etc/ssh/sshd_config

DESCRIPTION

sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
specified with -f on the command line). The file contains keyword-
argument pairs, one per line. Lines starting with `#' and empty lines
are interpreted as comments. Arguments may optionally be enclosed in
double quotes (") in order to represent arguments containing spaces.
The possible keywords and their meanings are as follows (note that
keywords are case-insensitive and arguments are case-sensitive):
AcceptEnv
Specifies what environment variables sent by the client will be
copied into the session's environ(7). See SendEnv in
ssh_config(5) for how to configure the client. Note that
environment passing is only supported for protocol 2. Variables
are specified by name, which may contain the wildcard characters
`*' and `?'. Multiple environment variables may be separated by
whitespace or spread across multiple AcceptEnv directives. Be
warned that some environment variables could be used to bypass
restricted user environments. For this reason, care should be
taken in the use of this directive. The default is not to accept
any environment variables.
AddressFamily
Specifies which address family should be used by sshd(8). Valid
arguments are ``any'', ``inet'' (use IPv4 only), or ``inet6''
(use IPv6 only). The default is ``any''.
AllowAgentForwarding
Specifies whether ssh-agent(1) forwarding is permitted. The
default is ``yes''. Note that disabling agent forwarding does
not improve security unless users are also denied shell access,
as they can always install their own forwarders.
AllowGroups
This keyword can be followed by a list of group name patterns,
separated by spaces. If specified, login is allowed only for
users whose primary group or supplementary group list matches one
of the patterns. Only group names are valid; a numerical group
ID is not recognized. By default, login is allowed for all
groups. The allow/deny directives are processed in the following
order: DenyUsers, AllowUsers, DenyGroups, and finally
AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
AllowTcpForwarding
Specifies whether TCP forwarding is permitted. The available
options are ``yes'' or ``all'' to allow TCP forwarding, ``no'' to
prevent all TCP forwarding, ``local'' to allow local (from the
perspective of ssh(1)) forwarding only or ``remote'' to allow
remote forwarding only. The default is ``yes''. Note that
disabling TCP forwarding does not improve security unless users
are also denied shell access, as they can always install their
own forwarders.
AllowUsers
This keyword can be followed by a list of user name patterns,
separated by spaces. If specified, login is allowed only for
user names that match one of the patterns. Only user names are
valid; a numerical user ID is not recognized. By default, login
is allowed for all users. If the pattern takes the form
USER@HOST then USER and HOST are separately checked, restricting
logins to particular users from particular hosts. The allow/deny
directives are processed in the following order: DenyUsers,
AllowUsers, DenyGroups, and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
AuthenticationMethods
Specifies the authentication methods that must be successfully
completed for a user to be granted access. This option must be
followed by one or more comma-separated lists of authentication
method names. Successful authentication requires completion of
every method in at least one of these lists.
For example, an argument of ``publickey,password
publickey,keyboard-interactive'' would require the user to
complete public key authentication, followed by either password
or keyboard interactive authentication. Only methods that are
next in one or more lists are offered at each stage, so for this
example, it would not be possible to attempt password or
keyboard-interactive authentication before public key.
For keyboard interactive authentication it is also possible to
restrict authentication to a specific device by appending a colon
followed by the device identifier ``bsdauth'', ``pam'', or
``skey'', depending on the server configuration. For example,
``keyboard-interactive:bsdauth'' would restrict keyboard
interactive authentication to the ``bsdauth'' device.
This option is only available for SSH protocol 2 and will yield a
fatal error if enabled if protocol 1 is also enabled. Note that
each authentication method listed should also be explicitly
enabled in the configuration. The default is not to require
multiple authentication; successful completion of a single
authentication method is sufficient.
AuthorizedKeysCommand
Specifies a program to be used to look up the user's public keys.
The program must be owned by root and not writable by group or
others. It will be invoked with a single argument of the
username being authenticated, and should produce on standard
output zero or more lines of authorized_keys output (see
AUTHORIZED_KEYS in sshd(8)). If a key supplied by
AuthorizedKeysCommand does not successfully authenticate and
authorize the user then public key authentication continues using
the usual AuthorizedKeysFile files. By default, no
AuthorizedKeysCommand is run.
AuthorizedKeysCommandUser
Specifies the user under whose account the AuthorizedKeysCommand
is run. It is recommended to use a dedicated user that has no
other role on the host than running authorized keys commands.
AuthorizedKeysFile
Specifies the file that contains the public keys that can be used
for user authentication. The format is described in the
AUTHORIZED_KEYS FILE FORMAT section of sshd(8).
AuthorizedKeysFile may contain tokens of the form %T which are
substituted during connection setup. The following tokens are
defined: %% is replaced by a literal '%', %h is replaced by the
home directory of the user being authenticated, and %u is
replaced by the username of that user. After expansion,
AuthorizedKeysFile is taken to be an absolute path or one
relative to the user's home directory. Multiple files may be
listed, separated by whitespace. The default is
``.ssh/authorized_keys .ssh/authorized_keys2''.
AuthorizedPrincipalsFile
Specifies a file that lists principal names that are accepted for
certificate authentication. When using certificates signed by a
key listed in TrustedUserCAKeys, this file lists names, one of
which must appear in the certificate for it to be accepted for
authentication. Names are listed one per line preceded by key
options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
Empty lines and comments starting with `#' are ignored.
AuthorizedPrincipalsFile may contain tokens of the form %T which
are substituted during connection setup. The following tokens
are defined: %% is replaced by a literal '%', %h is replaced by
the home directory of the user being authenticated, and %u is
replaced by the username of that user. After expansion,
AuthorizedPrincipalsFile is taken to be an absolute path or one
relative to the user's home directory.
The default is ``none'', i.e. not to use a principals file - in
this case, the username of the user must appear in a
certificate's principals list for it to be accepted. Note that
AuthorizedPrincipalsFile is only used when authentication
proceeds using a CA listed in TrustedUserCAKeys and is not
consulted for certification authorities trusted via
~/.ssh/authorized_keys, though the principals= key option offers
a similar facility (see sshd(8) for details).
Banner The contents of the specified file are sent to the remote user
before authentication is allowed. If the argument is ``none''
then no banner is displayed. This option is only available for
protocol version 2. By default, no banner is displayed.
ChallengeResponseAuthentication
Specifies whether challenge-response authentication is allowed.
All authentication styles from login.conf(5) are supported. The
default is ``yes''.
ChrootDirectory
Specifies the pathname of a directory to chroot(2) to after
authentication. All components of the pathname must be root-
owned directories that are not writable by any other user or
group. After the chroot, sshd(8) changes the working directory
to the user's home directory.
The pathname may contain the following tokens that are expanded
at runtime once the connecting user has been authenticated: %% is
replaced by a literal '%', %h is replaced by the home directory
of the user being authenticated, and %u is replaced by the
username of that user.
The ChrootDirectory must contain the necessary files and
directories to support the user's session. For an interactive
session this requires at least a shell, typically sh(1), and
basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4),
stderr(4), arandom(4) and tty(4) devices. For file transfer
sessions using ``sftp'', no additional configuration of the
environment is necessary if the in-process sftp server is used,
though sessions which use logging do require /dev/log inside the
chroot directory (see sftp-server(8) for details).
The default is not to chroot(2).
Ciphers
Specifies the ciphers allowed for protocol version 2. Multiple
ciphers must be comma-separated. The supported ciphers are
``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'',
``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'',
``aes128-gcm@openssh.com'', ``aes256-gcm@openssh.com'',
``arcfour128'', ``arcfour256'', ``arcfour'', ``blowfish-cbc'',
and ``cast128-cbc''. The default is:
aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-gcm@openssh.com,aes256-gcm@openssh.com,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour
ClientAliveCountMax
Sets the number of client alive messages (see below) which may be
sent without sshd(8) receiving any messages back from the client.
If this threshold is reached while client alive messages are
being sent, sshd will disconnect the client, terminating the
session. It is important to note that the use of client alive
messages is very different from TCPKeepAlive (below). The client
alive messages are sent through the encrypted channel and
therefore will not be spoofable. The TCP keepalive option
enabled by TCPKeepAlive is spoofable. The client alive mechanism
is valuable when the client or server depend on knowing when a
connection has become inactive.
The default value is 3. If ClientAliveInterval (see below) is
set to 15, and ClientAliveCountMax is left at the default,
unresponsive SSH clients will be disconnected after approximately
45 seconds. This option applies to protocol version 2 only.
ClientAliveInterval
Sets a timeout interval in seconds after which if no data has
been received from the client, sshd(8) will send a message
through the encrypted channel to request a response from the
client. The default is 0, indicating that these messages will
not be sent to the client. This option applies to protocol
version 2 only.
Compression
Specifies whether compression is allowed, or delayed until the
user has authenticated successfully. The argument must be
``yes'', ``delayed'', or ``no''. The default is ``delayed''.
DenyGroups
This keyword can be followed by a list of group name patterns,
separated by spaces. Login is disallowed for users whose primary
group or supplementary group list matches one of the patterns.
Only group names are valid; a numerical group ID is not
recognized. By default, login is allowed for all groups. The
allow/deny directives are processed in the following order:
DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
DenyUsers
This keyword can be followed by a list of user name patterns,
separated by spaces. Login is disallowed for user names that
match one of the patterns. Only user names are valid; a
numerical user ID is not recognized. By default, login is
allowed for all users. If the pattern takes the form USER@HOST
then USER and HOST are separately checked, restricting logins to
particular users from particular hosts. The allow/deny
directives are processed in the following order: DenyUsers,
AllowUsers, DenyGroups, and finally AllowGroups.
See PATTERNS in ssh_config(5) for more information on patterns.
ForceCommand
Forces the execution of the command specified by ForceCommand,
ignoring any command supplied by the client and ~/.ssh/rc if
present. The command is invoked by using the user's login shell
with the -c option. This applies to shell, command, or subsystem
execution. It is most useful inside a Match block. The command
originally supplied by the client is available in the
SSH_ORIGINAL_COMMAND environment variable. Specifying a command
of ``internal-sftp'' will force the use of an in-process sftp
server that requires no support files when used with
ChrootDirectory.
GatewayPorts
Specifies whether remote hosts are allowed to connect to ports
forwarded for the client. By default, sshd(8) binds remote port
forwardings to the loopback address. This prevents other remote
hosts from connecting to forwarded ports. GatewayPorts can be
used to specify that sshd should allow remote port forwardings to
bind to non-loopback addresses, thus allowing other hosts to
connect. The argument may be ``no'' to force remote port
forwardings to be available to the local host only, ``yes'' to
force remote port forwardings to bind to the wildcard address, or
``clientspecified'' to allow the client to select the address to
which the forwarding is bound. The default is ``no''.
GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed.
The default is ``no''. Note that this option applies to protocol
version 2 only.
GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's credentials
cache on logout. The default is ``yes''. Note that this option
applies to protocol version 2 only.
HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication
together with successful public key client host authentication is
allowed (host-based authentication). This option is similar to
RhostsRSAAuthentication and applies to protocol version 2 only.
The default is ``no''.
HostbasedUsesNameFromPacketOnly
Specifies whether or not the server will attempt to perform a
reverse name lookup when matching the name in the ~/.shosts,
~/.rhosts, and /etc/hosts.equiv files during
HostbasedAuthentication. A setting of ``yes'' means that sshd(8)
uses the name supplied by the client rather than attempting to
resolve the name from the TCP connection itself. The default is
``no''.
HostCertificate
Specifies a file containing a public host certificate. The
certificate's public key must match a private host key already
specified by HostKey. The default behaviour of sshd(8) is not to
load any certificates.
HostKey
Specifies a file containing a private host key used by SSH. The
default is /etc/ssh/ssh_host_key for protocol version 1, and
/etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key and
/etc/ssh/ssh_host_rsa_key for protocol version 2. Note that
sshd(8) will refuse to use a file if it is group/world-
accessible. It is possible to have multiple host key files.
``rsa1'' keys are used for version 1 and ``dsa'', ``ecdsa'' or
``rsa'' are used for version 2 of the SSH protocol. It is also
possible to specify public host key files instead. In this case
operations on the private key will be delegated to an
ssh-agent(1).
HostKeyAgent
Identifies the UNIX-domain socket used to communicate with an
agent that has access to the private host keys. If
``SSH_AUTH_SOCK'' is specified, the location of the socket will
be read from the SSH_AUTH_SOCK environment variable.
IgnoreRhosts
Specifies that .rhosts and .shosts files will not be used in
RhostsRSAAuthentication or HostbasedAuthentication.
/etc/hosts.equiv and /etc/shosts.equiv are still used. The
default is ``yes''.
IgnoreUserKnownHosts
Specifies whether sshd(8) should ignore the user's
~/.ssh/known_hosts during RhostsRSAAuthentication or
HostbasedAuthentication. The default is ``no''.
IPQoS Specifies the IPv4 type-of-service or DSCP class for the
connection. Accepted values are ``af11'', ``af12'', ``af13'',
``af21'', ``af22'', ``af23'', ``af31'', ``af32'', ``af33'',
``af41'', ``af42'', ``af43'', ``cs0'', ``cs1'', ``cs2'', ``cs3'',
``cs4'', ``cs5'', ``cs6'', ``cs7'', ``ef'', ``lowdelay'',
``throughput'', ``reliability'', or a numeric value. This option
may take one or two arguments, separated by whitespace. If one
argument is specified, it is used as the packet class
unconditionally. If two values are specified, the first is
automatically selected for interactive sessions and the second
for non-interactive sessions. The default is ``lowdelay'' for
interactive sessions and ``throughput'' for non-interactive
sessions.
KerberosAuthentication
Specifies whether the password provided by the user for
PasswordAuthentication will be validated through the Kerberos
KDC. To use this option, the server needs a Kerberos servtab
which allows the verification of the KDC's identity. The default
is ``no''.
KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT, attempt to
acquire an AFS token before accessing the user's home directory.
The default is ``no''.
KerberosOrLocalPasswd
If password authentication through Kerberos fails then the
password will be validated via any additional local mechanism
such as /etc/passwd. The default is ``yes''.
KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket
cache file on logout. The default is ``yes''.
KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms. Multiple
algorithms must be comma-separated. The default is
``ecdh-sha2-nistp256'', ``ecdh-sha2-nistp384'',
``ecdh-sha2-nistp521'', ``diffie-hellman-group-exchange-sha256'',
``diffie-hellman-group-exchange-sha1'',
``diffie-hellman-group14-sha1'', ``diffie-hellman-group1-sha1''.
KeyRegenerationInterval
In protocol version 1, the ephemeral server key is automatically
regenerated after this many seconds (if it has been used). The
purpose of regeneration is to prevent decrypting captured
sessions by later breaking into the machine and stealing the
keys. The key is never stored anywhere. If the value is 0, the
key is never regenerated. The default is 3600 (seconds).
ListenAddress
Specifies the local addresses sshd(8) should listen on. The
following forms may be used:
ListenAddresshost|IPv4_addr|IPv6_addrListenAddresshost|IPv4_addr:portListenAddress [host|IPv6_addr]:port
If port is not specified, sshd will listen on the address and all
prior Port options specified. The default is to listen on all
local addresses. Multiple ListenAddress options are permitted.
Additionally, any Port options must precede this option for non-
port qualified addresses.
LoginGraceTime
The server disconnects after this time if the user has not
successfully logged in. If the value is 0, there is no time
limit. The default is 120 seconds.
LogLevel
Gives the verbosity level that is used when logging messages from
sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
higher levels of debugging output. Logging with a DEBUG level
violates the privacy of users and is not recommended.
MACs Specifies the available MAC (message authentication code)
algorithms. The MAC algorithm is used in protocol version 2 for
data integrity protection. Multiple algorithms must be comma-
separated. The algorithms that contain ``-etm'' calculate the
MAC after encryption (encrypt-then-mac). These are considered
safer and their use recommended. The default is:
hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
umac-64-etm@openssh.com,umac-128-etm@openssh.com,
hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
hmac-md5-96-etm@openssh.com,
hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com,
hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,
hmac-sha1-96,hmac-md5-96
Match Introduces a conditional block. If all of the criteria on the
Match line are satisfied, the keywords on the following lines
override those set in the global section of the config file,
until either another Match line or the end of the file.
The arguments to Match are one or more criteria-pattern pairs.
The available criteria are User, Group, Host, LocalAddress,
LocalPort, and Address. The match patterns may consist of single
entries or comma-separated lists and may use the wildcard and
negation operators described in the PATTERNS section of
ssh_config(5).
The patterns in an Address criteria may additionally contain
addresses to match in CIDR address/masklen format, e.g.
``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask
length provided must be consistent with the address - it is an
error to specify a mask length that is too long for the address
or one with bits set in this host portion of the address. For
example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively.
Only a subset of keywords may be used on the lines following a
Match keyword. Available keywords are AcceptEnv,
AllowAgentForwarding, AllowGroups, AllowTcpForwarding,
AllowUsers, AuthenticationMethods, AuthorizedKeysCommand,
AuthorizedKeysCommandUser, AuthorizedKeysFile,
AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups,
DenyUsers, ForceCommand, GatewayPorts, GSSAPIAuthentication,
HostbasedAuthentication, HostbasedUsesNameFromPacketOnly,
KbdInteractiveAuthentication, KerberosAuthentication,
MaxAuthTries, MaxSessions, PasswordAuthentication,
PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTunnel,
PubkeyAuthentication, RekeyLimit, RhostsRSAAuthentication,
RSAAuthentication, X11DisplayOffset, X11Forwarding and
X11UseLocalHost.
MaxAuthTries
Specifies the maximum number of authentication attempts permitted
per connection. Once the number of failures reaches half this
value, additional failures are logged. The default is 6.
MaxSessions
Specifies the maximum number of open sessions permitted per
network connection. The default is 10.
MaxStartups
Specifies the maximum number of concurrent unauthenticated
connections to the SSH daemon. Additional connections will be
dropped until authentication succeeds or the LoginGraceTime
expires for a connection. The default is 10:30:100.
Alternatively, random early drop can be enabled by specifying the
three colon separated values ``start:rate:full'' (e.g.
"10:30:60"). sshd(8) will refuse connection attempts with a
probability of ``rate/100'' (30%) if there are currently
``start'' (10) unauthenticated connections. The probability
increases linearly and all connection attempts are refused if the
number of unauthenticated connections reaches ``full'' (60).
PasswordAuthentication
Specifies whether password authentication is allowed. The
default is ``yes''.
PermitEmptyPasswords
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password strings. The
default is ``no''.
PermitOpen
Specifies the destinations to which TCP port forwarding is
permitted. The forwarding specification must be one of the
following forms:
PermitOpenhost:portPermitOpenIPv4_addr:portPermitOpen[IPv6_addr]:port
Multiple forwards may be specified by separating them with
whitespace. An argument of ``any'' can be used to remove all
restrictions and permit any forwarding requests. An argument of
``none'' can be used to prohibit all forwarding requests. By
default all port forwarding requests are permitted.
PermitRootLogin
Specifies whether root can log in using ssh(1). The argument
must be ``yes'', ``without-password'', ``forced-commands-only'',
or ``no''. The default is ``yes''.
If this option is set to ``without-password'', password
authentication is disabled for root.
If this option is set to ``forced-commands-only'', root login
with public key authentication will be allowed, but only if the
command option has been specified (which may be useful for taking
remote backups even if root login is normally not allowed). All
other authentication methods are disabled for root.
If this option is set to ``no'', root is not allowed to log in.
PermitTunnel
Specifies whether tun(4) device forwarding is allowed. The
argument must be ``yes'', ``point-to-point'' (layer 3),
``ethernet'' (layer 2), or ``no''. Specifying ``yes'' permits
both ``point-to-point'' and ``ethernet''. The default is ``no''.
PermitUserEnvironment
Specifies whether ~/.ssh/environment and environment= options in
~/.ssh/authorized_keys are processed by sshd(8). The default is
``no''. Enabling environment processing may enable users to
bypass access restrictions in some configurations using
mechanisms such as LD_PRELOAD.
PidFile
Specifies the file that contains the process ID of the SSH
daemon. The default is /var/run/sshd.pid.
Port Specifies the port number that sshd(8) listens on. The default
is 22. Multiple options of this type are permitted. See also
ListenAddress.
PrintLastLog
Specifies whether sshd(8) should print the date and time of the
last user login when a user logs in interactively. The default
is ``yes''.
PrintMotd
Specifies whether sshd(8) should print /etc/motd when a user logs
in interactively. (On some systems it is also printed by the
shell, /etc/profile, or equivalent.) The default is ``yes''.
Protocol
Specifies the protocol versions sshd(8) supports. The possible
values are `1' and `2'. Multiple versions must be comma-
separated. The default is `2'. Note that the order of the
protocol list does not indicate preference, because the client
selects among multiple protocol versions offered by the server.
Specifying ``2,1'' is identical to ``1,2''.
PubkeyAuthentication
Specifies whether public key authentication is allowed. The
default is ``yes''. Note that this option applies to protocol
version 2 only.
RekeyLimit
Specifies the maximum amount of data that may be transmitted
before the session key is renegotiated, optionally followed a
maximum amount of time that may pass before the session key is
renegotiated. The first argument is specified in bytes and may
have a suffix of `K', `M', or `G' to indicate Kilobytes,
Megabytes, or Gigabytes, respectively. The default is between
`1G' and `4G', depending on the cipher. The optional second
value is specified in seconds and may use any of the units
documented in the TIMEFORMATS section. The default value for
RekeyLimit is ``default none'', which means that rekeying is
performed after the cipher's default amount of data has been sent
or received and no time based rekeying is done. This option
applies to protocol version 2 only.
RevokedKeys
Specifies revoked public keys. Keys listed in this file will be
refused for public key authentication. Note that if this file is
not readable, then public key authentication will be refused for
all users. Keys may be specified as a text file, listing one
public key per line, or as an OpenSSH Key Revocation List (KRL)
as generated by ssh-keygen(1). For more information on KRLs, see
the KEY REVOCATION LISTS section in ssh-keygen(1).
RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication
together with successful RSA host authentication is allowed. The
default is ``no''. This option applies to protocol version 1
only.
RSAAuthentication
Specifies whether pure RSA authentication is allowed. The
default is ``yes''. This option applies to protocol version 1
only.
ServerKeyBits
Defines the number of bits in the ephemeral protocol version 1
server key. The minimum value is 512, and the default is 1024.
StrictModes
Specifies whether sshd(8) should check file modes and ownership
of the user's files and home directory before accepting login.
This is normally desirable because novices sometimes accidentally
leave their directory or files world-writable. The default is
``yes''. Note that this does not apply to ChrootDirectory, whose
permissions and ownership are checked unconditionally.
Subsystem
Configures an external subsystem (e.g. file transfer daemon).
Arguments should be a subsystem name and a command (with optional
arguments) to execute upon subsystem request.
The command sftp-server(8) implements the ``sftp'' file transfer
subsystem.
Alternately the name ``internal-sftp'' implements an in-process
``sftp'' server. This may simplify configurations using
ChrootDirectory to force a different filesystem root on clients.
By default no subsystems are defined. Note that this option
applies to protocol version 2 only.
SyslogFacility
Gives the facility code that is used when logging messages from
sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0,
LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
default is AUTH.
TCPKeepAlive
Specifies whether the system should send TCP keepalive messages
to the other side. If they are sent, death of the connection or
crash of one of the machines will be properly noticed. However,
this means that connections will die if the route is down
temporarily, and some people find it annoying. On the other
hand, if TCP keepalives are not sent, sessions may hang
indefinitely on the server, leaving ``ghost'' users and consuming
server resources.
The default is ``yes'' (to send TCP keepalive messages), and the
server will notice if the network goes down or the client host
crashes. This avoids infinitely hanging sessions.
To disable TCP keepalive messages, the value should be set to
``no''.
TrustedUserCAKeys
Specifies a file containing public keys of certificate
authorities that are trusted to sign user certificates for
authentication. Keys are listed one per line; empty lines and
comments starting with `#' are allowed. If a certificate is
presented for authentication and has its signing CA key listed in
this file, then it may be used for authentication for any user
listed in the certificate's principals list. Note that
certificates that lack a list of principals will not be permitted
for authentication using TrustedUserCAKeys. For more details on
certificates, see the CERTIFICATES section in ssh-keygen(1).
UseDNS Specifies whether sshd(8) should look up the remote host name and
check that the resolved host name for the remote IP address maps
back to the very same IP address. The default is ``yes''.
UseLogin
Specifies whether login(1) is used for interactive login
sessions. The default is ``no''. Note that login(1) is never
used for remote command execution. Note also, that if this is
enabled, X11Forwarding will be disabled because login(1) does not
know how to handle xauth(1) cookies. If UsePrivilegeSeparation
is specified, it will be disabled after authentication.
UsePrivilegeSeparation
Specifies whether sshd(8) separates privileges by creating an
unprivileged child process to deal with incoming network traffic.
After successful authentication, another process will be created
that has the privilege of the authenticated user. The goal of
privilege separation is to prevent privilege escalation by
containing any corruption within the unprivileged processes. The
default is ``yes''. If UsePrivilegeSeparation is set to
``sandbox'' then the pre-authentication unprivileged process is
subject to additional restrictions.
VersionAddendum
Optionally specifies additional text to append to the SSH
protocol banner sent by the server upon connection. The default
is ``none''.
X11DisplayOffset
Specifies the first display number available for sshd(8)'s X11
forwarding. This prevents sshd from interfering with real X11
servers. The default is 10.
X11Forwarding
Specifies whether X11 forwarding is permitted. The argument must
be ``yes'' or ``no''. The default is ``no''.
When X11 forwarding is enabled, there may be additional exposure
to the server and to client displays if the sshd(8) proxy display
is configured to listen on the wildcard address (see
X11UseLocalhost below), though this is not the default.
Additionally, the authentication spoofing and authentication data
verification and substitution occur on the client side. The
security risk of using X11 forwarding is that the client's X11
display server may be exposed to attack when the SSH client
requests forwarding (see the warnings for ForwardX11 in
ssh_config(5)). A system administrator may have a stance in
which they want to protect clients that may expose themselves to
attack by unwittingly requesting X11 forwarding, which can
warrant a ``no'' setting.
Note that disabling X11 forwarding does not prevent users from
forwarding X11 traffic, as users can always install their own
forwarders. X11 forwarding is automatically disabled if UseLogin
is enabled.
X11UseLocalhost
Specifies whether sshd(8) should bind the X11 forwarding server
to the loopback address or to the wildcard address. By default,
sshd binds the forwarding server to the loopback address and sets
the hostname part of the DISPLAY environment variable to
``localhost''. This prevents remote hosts from connecting to the
proxy display. However, some older X11 clients may not function
with this configuration. X11UseLocalhost may be set to ``no'' to
specify that the forwarding server should be bound to the
wildcard address. The argument must be ``yes'' or ``no''. The
default is ``yes''.
XAuthLocation
Specifies the full pathname of the xauth(1) program. The default
is /usr/X11R6/bin/xauth.

TIME FORMATS

sshd(8) command-line arguments and configuration file options that
specify time may be expressed using a sequence of the form:
time[qualifier], where time is a positive integer value and qualifier is
one of the following:
<none> seconds
s | S seconds
m | M minutes
h | H hours
d | D days
w | W weeks
Each member of the sequence is added together to calculate the total time
value.
Time format examples:
600 600 seconds (10 minutes)
10m 10 minutes
1h30m 1 hour 30 minutes (90 minutes)

FILES

/etc/ssh/sshd_config
Contains configuration data for sshd(8). This file should be
writable by root only, but it is recommended (though not
necessary) that it be world-readable.

SEE ALSO

AUTHORS

OpenSSH is a derivative of the original and free ssh 1.2.12 release by
Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
de Raadt and Dug Song removed many bugs, re-added newer features and
created OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
for privilege separation.
OpenBSD 5.4 July 19, 2013 OpenBSD 5.4